1. Field of the Invention
The present invention relates to an image forming apparatus for reproducing original document images by an electrophotographic process using at least a single developing means for color development. More specifically, the present invention relates to an electrophotographic image forming apparatus capable of fixing a toner image by means of a fixing means having a set of rollers.
2. Description of the Related Art
Electrophotographic processes comprise a charging process to uniformly impart an electrical charge to the surface of a photosensitive member, an exposure process to partially discharge the electrical charge on the surface of the photosensitive member by irradiating said photosensitive member with light so as to form an electrostatic latent image thereon in accordance with the image information, a developing process for adhering toner contained in a developing material onto the aforesaid latent image so as to form a toner image, a transfer process for transferring the toner image to a recording paper sheet (hereinafter referred to as paper sheet), and a fixing process to fuse the transferred toner image to the paper by heating, and are widely used as a hardcopy image forming technique.
Copying machine using electrophotographic processes generally is capable of using transparency sheets such as those used by overhead projectors (OHP) in addition to plan paper sheets (non-transparencies).
When OHP copy images are formed containing red, blue or like colors, the light transmitting ability of the transparency must be assured by completely fusing the color toner to the surface of the transparency sheet in such a way that the surface of the sheet remains flat, thereby enhancing the reproducibility of the particular hue.
A typical method for assuring light transmitting ability of transparency sheets is to use in the fixing process a comparatively longer fixing time for the transparency than the time used for plain paper fixing. This method is safer than a method wherein the fixing means uses a heating temperature that is higher than normal.
When the OHP mode is selected by an operation key or the like, a conventional copying machine is constructed so as to be capable of executing the fixing process alone from among the previously mentioned electrophotographic processes on the supplied transparency. That is, the fixing process can be executed a plurality of times on a single transparent resin sheet.
When obtaining an OHP copy image from a conventional copying machine, the operator usually first selects the copy mode (the fixing time being identical to the fixing time for plain paper copying), and after fixing, the fixed transparency sheet is discharged with the color toner image corresponding to the original document image in a semi-fused state.
Then, after the operator selects the OHP mode, the previously discharged transparency sheet is manually fed into the copying machine and the color toner image is subjected a second fixing process wherein fusion of the aforesaid color toner image is completed to assure the light transmitting ability of the sheet.
Copying machines have been proposed which automatically select the OHP mode when a transparency sheet is detected, and the fixing time in the OHP mode is prolonged by retarding the rotational speed of the heating roller of the fixing means (U.S. Pat. No. 4,549,803).
The conventional copying machine capable of multiple executions of the fixing process in the manner described above requires that the operator must manually feed the temporarily discharged transparency sheet, thereby presenting a disadvantage in that the operator must perform an additional operational task.
Further, in the electrophotographic process, the transport speed of the entire surface of the transport sheet must be held uniform so as to prevent disruption (Deterioration of image quality) of the toner image caused by the flexibility of the sheet in the sheet transport direction.
Accordingly, when the fixing roller rotational speed is retarded, the rotational speed of the photosensitive member or transfer belt must be retarded in conjunction with the slowdown in the rotational speed of the fixing roller, or the distance between the transfer position and the fixing position must be longer than the length of the transfer sheet.
However, when the rotational speed of the transfer belt or the like is retarded, the exposure speed and developing speed must also be retarded in conjunction with the slowdown of the rotational speed of the transfer belt or the like. The operational speeds of all electrophotographic processes are dissimilar in the normal copy mode and the OHP mode. The operational controls therefore become more complex, and it is difficult to obtain optimal electrophotographic processing conditions (physical property values) suited to the respective operational speeds for normal mode and OHP mode, particularly immediately after mode switching, thereby resulting in a further disadvantage of a loss of stability in image quality.
When the distance between the transfer position and the fixing position is lengthened in accordance with the sheet size, the range of paper sizes that are be accommodated is broadened, for example, from B4 size sheets to A3 size sheets. Further disadvantage result therefrom inasmuch as the overall size of the copying machine becomes larger with a proportional increase in cost.
In addition, when the OHP mode is selected, the fixing process is executed at low speed even if the copy image is formed in black color toner only which does not require light transmitting ability. This arrangement results in still another disadvantage in that the time required for the copying process is unnecessarily prolonged.
The fixing means used to accomplish the fixing process in electrophotographic copying machines typically has top roller and a bottom roller that respectively make contact with the front surface (surface upon which the toner image is formed) and reverse surface of the copy sheet, and is constructed so as to apply heat of a specified temperature to these rollers by means of a heating lamp or similar heating means. The toner image formed on the front surface of the sheet passes between the aforesaid heated rollers and is fused thereon so as to be fixed to the surface of the copy sheet.
Regulation of the temperatures of the top and bottom fixing rollers, i.e., regulation of the fixing temperature, is controlled by detecting the surface temperature of each roller by means of temperature sensors, and supplying electrical power (ON) to the heating means when the surface temperature is less than a set temperature, and interrupting the supply of electrical power (OFF) to the heating means when the surface temperature exceeds a set temperature.
In conventional copying machines temperature regulation is typically executed for both the top fixing roller and bottom fixing roller whether in the normal copy mode or the OHP mode. That is, the heating means of each roller is individually controlled so as to switch ON/OFF according to the output from the respective temperature sensors. When the temperature of an individual roller becomes less than a set temperature, electrical power for heating is supplied to the individual roller concerned.
Therefore, a disadvantage arises inasmuch as the electrical power supplied to the fixing means is not used efficiently with regard to the overall power consumption of the copying machine.
In other words, when forming an OHP copy image incorporating color, the color toner on the transparency sheet must be completely fused thereon to retain the flatness of the sheet surface and assure the light transmitting ability so as to thereby enhance the reproducibility of the color as previously described. Accordingly, in the OHP mode, heating efficiency is most efficacious when applied from both the front and the rear of the toner image on the transparency sheet. More specifically, the direct heating application by the top roller on the toner image and the indirect heating application by the bottom roller through the transparency sheet and the temperature may be regulated for both the top roller and the bottom roller.
In contrast, direct heating of the toner image is sufficient in the normal copy mode because the copy image is not adversely affected even if the toner image is fixed in a semi-fused state. Further, the normal copy mode is frequently used to make multiple copies wherein the image of a single document repeatedly reproduced on a plurality of copy sheets, and using all the electrical power supplied to both rollers to regulate the temperature of the top roller is most efficient in assuring a specific fixing temperature even when making multicopies. Thus, conventional copying machines which supply electrical power to both fixing rollers when making multiple copies of a single document are inefficiently supplying said power.
In copying machines that use commercial power supplies (100V), the maximum power consumption is set at less than 1.5 kW so as to allow the use of normal indoor wiring (rated for current of 15 amps).